Translation from the nanoscale into macroscale structures of the outstanding thermal, electrical, mechanical, and fluidic properties encountered in many nanomaterials promises to offer new solutions to long-standing challenges in materials science and technology. For membrane applications, materials with properties to overcome the typical trade-off between permeability and selectivity could provide major breakthroughs in several areas, from efficient water purification and energy harvesting, to low-cost separation of high-value chemicals, pharmaceuticals, and biological components.
Recent studies demonstrated that carbon nanotubes (CNTs) enable liquid and gas transport rates within their narrow core that are several orders of magnitude larger than expected for nanopores of similar sizes. Thus, (sub-) nanometer CNT pores have great potential for transforming separation applications if large-scale CNT membranes can be assembled. This ability of CNTs to sustain ultrafast rates of fluid transport is especially promising for the so-far unexplored yet critical field of breathable and protective fabrics. Recent world events, including viral epidemics (Ebola, SARS, avian flu) and the release of chemical warfare agents (sarin, sulfur mustard) in the ongoing Syria conflict, have highlighted the urgency to develop materials that can protect against hazardous agents to ensure the safety of civilian, medical, and military personnel.
To minimize physiological burden and prevent the risk of heat stress, a protective garment also has to allow facile perspiration and efficient heat loss from the body by evaporative cooling. Unfortunately, achieving both high protection and breathability (i.e. rapid water vapor transport) in a single material remains elusive. Current protective materials sacrifice breathability in order to prevent exposure to harmful agents. There are typically either impermeable barriers that entirely block penetration of chemical and biological hazards (but also of water vapor), or heavy-weight laminates containing adsorbents for harmful agents. Conversely, macroporous membranes with high permeability to moisture vapor and air offer poor protection. Indeed, because their ability to protect typically relies on hydrophobicity/oleophobicity, low-tension liquids can penetrate their porous network structure and potentially shuttle in other hazardous components. Furthermore, these macroporous materials are ineffective against vapor-phase threats.
Recent approaches to achieve adequate breathability in protective materials typically encompass selective monolithic membranes made of novel hydrophilic polymers, or multifunctional materials containing chemical groups/oxide nanoparticles with antibacterial or self-decontamination ability. An alternative route with truly transformative potential requires designing/fabricating smart dynamic materials that exhibit a reversible, rapid transition from a breathable state to a protective state triggered by environmental threats. These responsive membranes are expected to be particularly effective in mitigating physiological burden because a less breathable but protective state can be actuated locally and only when needed.